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The most common cause of the check hybrid system warning on the camry is a problem with the hybrid battery pack. It's not always the case though so the best thing to do is to have the vehicle scanned for trouble codes so you will know which system is telling the vehicles' computer to display the warning message.
I have seen the check hybrid warning come on for a bad hybrid battery coolant pump, bad hybrid battery, bad hybrid battery controller, etc. so it's not really possible to determine what is wrong without a little more information. Do you have any symptoms? Does the gas engine run constantly? Does it seem to run poorly?